Using the granular product of this invention, the alkaline earth metal carbonate raw materials in the glass batch react more readily thereby increasing the production capacity of the glass furnace.
Alkaline earth metal carbonates, as well as alkali metal carbonates, are ingredients in some glass formulations. In the production of the face plates for color television sets, strontium carbonate and barium carbonate constitute a sizeable portion of the glass furnace feed. Strontium carbonate makes up about 15% of the weight of the glass furnace feed for color TV faceplate glass; barium carbonate is usually present in equal or lesser amounts. These strontium and barium ions function in the glass as X-ray absorbers. Lesser amounts of calcium carbonate and magnesium carbonate are usually added to the glass furnace feed also. The face plates of black-and-white televisions contain barium carbonate as the primary X-ray absorber.
In the predominant commercial production method for strontium carbonate or barium carbonate, naturally occurring strontium sulfate (celestite) or barium sulfate (barite) is reduced to strontium sulfide or barium sulfide; these are water soluble. The sulfide is dissolved in water, insoluble impurities are removed by filtration, and either sodium carbonate solution or carbon dioxide gas is introduced into the solution to precipitate substantially pure crystals of insoluble strontium carbonate or barium carbonate. These very small crystals yield fluffy, low bulk density powders which are not free flowing.
Another production method for strontium carbonate entails reacting the naturally occurring strontium sulfate with sodium carbonate in aqueous solution to form sodium sulfate and strontium carbonate. The sodium sulfate has a much higher solubility in water than strontium carbonate, so filtration and washing will yield a solid strontium carbonate essentially free of sodium sulfate. Once again, this method yields a fine, fluffy, low bulk density powder which is not free flowing.
Calcium carbonate and magnesium carbonate for use as feed-stock for glass manufacture are usually produced by crushing and grinding naturally occurring mineral of the required purity to the required particle size (smaller than about 1 millimeter). They, like barium carbonate and strontium carbonate, are essentially insoluble in water.
Strontium carbonate and barium carbonate products suited for feedstock for glass manufacture have previously been produced primarily by heating the small crystals to a temperature of about 800.degree. C. for sufficient time to allow sintering of the crystals to occur. This process yields a dense, coarse granular material that must be ground to a particle size acceptable to the glass manufacturer (below about 1 millimeter particle size). Impurities derived from the heat treatment process, usually refractory brick pieces and/or iron scale, contaminate sintered products and are detrimental to glass furnace performance. U.S. Pat. Nos. 3,802,901 and 3,883,364 (Robertson et al.) describe an improvement to the sintering process entailing the admixture of an aqueous sodium silicate solution with the small crystals to place up to 0.50 percent by weight of silicate (expressed as silicon dioxide) in the mixture, based on the dry weight of the alkaline earth metal carbonate to allow sintering to proceed more readily.
Spray drying to form spherical particles of a size suitable for a glass furnace feedstock as well as briquetting of the strontium carbonate or barium carbonate powder under high pressure followed by crushing have been employed to produce glass-grade products (with particle size below about 1 millimeter and above about 0.1 millimeter). U.S. Pat. Nos. 4,88,161 and 4,888,308 (Adams, Jr. et al.) teach that fully dispersed, spray dried spheres can be heat treated to a temperature below the sintering temperature to increase their bulk density and thus make them more acceptable for glass furnace feed-stock.
In the manufacture of glass objects, glass raw materials such as sand, soda ash (sodium carbonate), potash (potassium carbonate), lime (calcium carbonate), barium carbonate, and strontium carbonate are usually accurately weighed into a batch mixer, intimately admixed to form the "glass batch", then introduced into a continuous tank furnace in which the fuel (usually natural gas) is burned inside the tank itself in tremendous flames that shoot out over the top of a pool of molten glass and unmelted or undissolved raw materials. The raw material feed components should be granular to prevent their being blown out of the glass furnace. At some temperature and after some length of time the carbonates react with the silica sand liberating carbon dioxide gas (if nitrates are added to the glass batch, oxides of nitrogen will also be liberated). The gas bubbles escape and a homogeneous melt is achieved if the molten glass is held in the glass furnace for a sufficiently long time. The molten glass is then allowed to pour from the furnace and it is formed into the desired shape.
In producing television face plates, quality is critical. If the slightest imperfection, such as an undissolved particle, a tiny gas bubble, or a "cold glass defect" caused by inhomogenity or incomplete melting is detected, the face plate must be broken and remelted in the glass furnace. The capacity of any particular glass furnace producing television face plate glass is determined by the ease of melting of the glass raw materials and/or the percentage of the glass product which is flawed and must be fed back to the furnace for remelting. Sources of alkali metal ions which are more expensive than sodium carbonate and potassium carbonate are sometimes used in limited quantities to promote the melting of the glass batch. A glass batch composed of granular raw materials which will react together rapidly upon heating to form a homogeneous glass free of defects greatly improves the economics of glass manufacture.
Glass-making involves a series of poorly understood and complex steps involving melting, dissolution, decomposition, and chemical reaction. A simplistic model of the process, occurring between granules of sand, alkali metal carbonates, and alkaline earth metal carbonates, would involve:
1. The lowest melting components becoming liquids: PA1 2. The liquid phase coating the surfaces of the granules of higher melting components: sand [M.P. 1601.degree. C.], calcium carbonate [M.P. about 1300.degree. C.], strontium carbonate [M.P. about 1200.degree. C.], barium carbonate [M.P. about 1300.degree. C.], and broken recycled glass [M.P. about 1050.degree. C.]; PA1 3. Some of the liquid phase possibly draining down to the bottom of the raw material bed creating inhomogeneity; PA1 4. The liquid reacting with the surface of the sand grains EQU Na.sub.2 CO.sub.3 +SiO.sub.2 .fwdarw.Na.sub.2 SiO.sub.3 +CO.sub.2 (gas) EQU K.sub.2 CO.sub.3 +SiO.sub.2 .fwdarw.K.sub.2 SiO.sub.3 +CO.sub.2 (gas) PA1 5. The alkaline earth metal carbonates eventually melting or dissolving to become a part of the liquid phase, then reacting with the surface of the sand grains EQU SrCO.sub.3 +SiO.sub.2 .fwdarw.SrSiO.sub.3 +CO.sub.2 (gas) EQU BaCO.sub.3 +SiO.sub.2 .fwdarw.BaSiO.sub.3 +CO.sub.2 (gas) PA1 6. The liquid continuing to react with the sand (for example, Na.sub.2 SiO.sub.3 +SiO.sub.2 .fwdarw.Na.sub.2 Si.sub.2 O.sub.5) and the gas bubbles in the liquid slowly rising to be released at the surface of the molten glass. PA1 7. Diffusion and convection currents within the liquid resulting in a homogeneous glass composition after all solids have melted and/or reacted. PA1 a) forming an admixture of fine alkaline earth metal carbonate particles (preferably smaller than about 0.04 millimeter diameter) and water sufficient to at least form a damp mass; PA1 c) mixing the admixture of alkaline earth metal carbonate, water, and alkali metal salt for sufficient time to allow at least a portion of the alkali metal salt to dissolve in the aqueous phase and to achieve homogeneity; PA1 d) drying and, if necessary, crushing the dried product to yield granules that are of a size acceptable for glass manufacture, usually about 1 millimeter or less in diameter.
sodium carbonate melts at 854.degree. C. PA2 potassium carbonate melts at 901.degree. C.
and the alkaline earth metal carbonate granules beginning to dissolve in the liquid.
All of this occurs while the temperature within the glass furnace is raised to about 1300.degree. C. The raw materials introduced to the glass furnace are in the form of particles averaging about 0.5 millimeter in diameter with none larger than about 1 millimeter. This particle size promotes ease of materials handling (free flowing grains), intimate admixing of the glass batch ingredients, and minimizes dust generation.
To address the problem of alkali metal salts such as sodium and potassium carbonate melting at relatively low temperatures and draining away from the other ingredients in a glass batch causing inhomogeneity in the glass furnace, U.S. Pat. 3,817,776 (Gringras, "Granular Free-Flowing Material for Use in the Manufacture of Glass") teaches the spraying of sodium hydroxide solution onto sand grains then heating to 320.degree. C. to 450.degree. C. to allow reaction of the caustic with the surface of the sand. This reaction transforms the sodium hydroxide into sodium metasilicate (melting point 1088.degree. C.) according to the patent.